NuSTAR Observations Research Articles

An investigation of the ‘10 keV feature’ in the spectra of accretion powered X-ray pulsars with NuSTAR

ABSTRACT Some of the accreting X-ray pulsars are reported to exhibit a peculiar spectral feature at ∼10 keV, known as the ‘10 keV feature’. The feature has been characterized as either an emission line or an absorption line, and its origin is unknown. It has been found in multiple observations of the same source by different observatories, but not all the observations of any particular source consistently showed the presence of it. In this work, we have carried out a systematic investigation for the presence of the ‘10 keV feature’ using data from NuSTAR, a low background spectroscopic observatory having uninterrupted wide band coverage on either side of 10 keV. We performed a systematic spectral analysis on 58 archival NuSTAR observations of 30 bright X-ray pulsars. The 3–79 keV spectral continua of these selected sources were fitted with a model chosen on the basis of its fitting quality in 3–15 keV and model simplicity, and then inspected for the presence of the ‘10 keV feature’. Our analysis indicates the presence of such a feature in 16 out of 58 the NuSTAR observations of 11 different sources and is fitted with a Gaussian absorption model centred around 10 keV. Our analysis also suggests that such a feature could be wrongly detected if flare data is not analysed separately from persistent emission.

NuSTAR Observations of Four Mid-IR–Selected Dual AGN Candidates in Galaxy Mergers

Mergers of galaxies are a ubiquitous phenomenon in the universe and represent a natural consequence of the “bottom-up” mass accumulation and galaxy evolution cosmological paradigm. It is generally accepted that the peak of active galactic nucleus (AGN) accretion activity occurs at nuclear separations of ≲10 kpc for major mergers. Here we present new NuSTAR and XMM-Newton observations for a subsample of mid-IR preselected dual AGN candidates in an effort to better constrain the column densities along the line of sight (LOS) for each system. Only one dual AGN candidate, J0841+0101, is detected as a single, unresolved source in the XMM-Newton and NuSTAR imaging, while the remaining three dual AGN candidates, J0122+0100, J1221+1137, and J1306+0735, are not detected with NuSTAR; if these nondetections are due to obscuration alone, these systems are consistent with being absorbed by column densities of log(N H/cm−2) ≥ 24.9, 24.6, and 24.3, which are roughly consistent with previously inferred column densities in these merging systems. In the case of J0841+0101, the analysis of the 0.3–30 keV spectra reveal an LOS column density of N H ≳ 1024 cm−2, significantly larger than the column densities previously reported for this system and demonstrating the importance of the higher signal-to-noise ratio XMM-Newton spectra and access to the >10 keV energies via NuSTAR. Though it is unclear if J0841+0101 truly hosts a dual AGN, these results are in agreement with the high obscuring columns expected in AGNs in late-stage mergers.

NuSTAR Observations of Abell 665 and 2146: Constraints on Nonthermal Emission

Observations from past missions such as RXTE and Beppo-SAX suggested the presence of inverse Compton (IC) scattering at hard X-ray energies within the intracluster medium of some massive galaxy clusters. In subsequent years, observations by, e.g., Suzaku, and now NuSTAR, have not been able to confirm these detections. We report on NuSTAR hard X-ray searches for IC emission in two massive galaxy clusters, A665 and A2146. To constrain the global IC flux in these two clusters, we fit global NuSTAR spectra with three models: single (1T) and two-temperature (2T) models, and a 1T plus power-law component (T+IC). The temperature components are meant to characterize the thermal ICM emission, while the power law represents the IC emission. We find that the 3–30 keV A665 and 3–20 keV A2146 spectra are best described by thermal emission alone, with average global temperatures of kT = (9.15 ± 0.1) keV for A665 and kT = (8.29 ± 0.1) keV for A 2146. We constrain the IC flux to F NT <0.60 × 10−12 erg s−1 cm−2 and F NT < 0.85 × 10−12 erg s−1 cm−2 (20–80 keV) for A665 and A2146, respectively both at the 90% confidence level. When we couple the IC flux limits with 1.4 GHz diffuse radio data from the VLA, we set lower limits on the average magnetic field strengths of >0.14 μG and >0.011 μG for A665 and A2146, respectively.

An Extreme Black Hole in the Recurrent X-Ray Transient XTE J2012+381

Abstract The black hole (BH) candidate XTE J2012+381 underwent an outburst at the end of 2022. We analyzed 105 NICER observations and two NuSTAR observations of the source during the outburst. The NuSTAR observations of the M ∼ 10 M ⊙ BH indicate clear signs of relativistic disk reflection, which we modeled to measure a BH spin of a = 0.988 − 0.030 + 0.008 and an inclination of θ = 68 − 11 + 6 deg (1σ statistical errors). In our analysis, we test an array of models and examine the effect of fitting NuSTAR spectra alone versus fitting simultaneously with NICER. We find that when the underlying continuum emission is properly accounted for, the reflected emission is similarly characterized by multiple models. We combined 52 NICER spectra to obtain a spectrum with an effective exposure of 190 ks in order to probe the presence of absorption lines that would be suggestive of disk winds, but the resulting features were not statistically significant. We discuss the implications of this measurement in relation to the overall BH spin distribution in X-ray binary systems.

Compton-thick AGN in the NuSTAR Era X: Analysing seven local CT-AGN candidates

Context. We present the broad-band X-ray spectral analysis (0.6–50 keV) of seven Compton-thick active galactic nuclei (CT-AGN; line-of-sight (LOS) column density &gt; 1024 cm−2) candidates selected from the Swift-BAT 100 month catalogue using archival NuSTAR data. Aims. We aim to obtain a complete census of the heavily obscured AGN in the local Universe (z &lt; 0.05). Methods. This work is in continuation of the ongoing research of the Clemson-INAF group to classify CT-AGN candidates at redshift z &lt; 0.05 using physically motivated torus models. Results. Our results confirm that three out of seven targets are bona fide CT-AGN. Adding our results to the previously analysed sources using NuSTAR data, we increase the population of bona fide CT-AGN by ∼9%, bringing the total number to 35 out of 414 AGN. We also performed a comparative study using MYTorus and borus02 on the spectra in our sample, finding that both physical models are strongly consistent in the parameter space of LOS column density and photon index. Furthermore, we also investigate the clumpiness of the torus clouds by separately computing the LOS and average torus column densities in each of the seven sources. Adding our results to all the previous 48 CT-AGN candidates analysed by the Clemson-INAF research team for which NuSTAR observations are available, we find that 78% of the sources are likely to have a clumpy distribution of the obscuring material surrounding the accreting supermassive black hole.

Detection of a strong ~2.5 Hz modulation in the newly discovered millisecond pulsar MAXI J1816–195

ABSTRACT MAXI J1816–195 is a newly discovered accreting millisecond X-ray pulsar that went outburst in 2022 June. Through timing analysis with Neutron star Interior Composition Explorer (NICER) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations, we find a transient modulation at ~2.5 Hz during the decay period of MAXI J1816–195. The modulation is strongly correlated with a spectral hardening, and its fractional rms amplitude increases with energy. These results suggest that the modulation is likely to be produced in an unstable corona. In addition, the presence of the modulation during thermonuclear bursts indicates that it may originate from a disc-corona where the optical depth is likely the main factor affecting the modulation, rather than temperature. Moreover, we find significant reflection features in the spectra observed simultaneously by NICER and NuSTAR, including a relativistically broadened Fe-K line around 6–7 keV, and a Compton hump in the 10–30 keV energy band. The radius of the inner disc is constrained to be Rin = (1.04–1.23) RISCO based on reflection modeling of the broad-band spectra. Assuming that the inner disc is truncated at the magnetosphere radius, we estimate that the magnetic field strength is $\le 4.67 \times 10^{8}\, \rm G$.

The complex spectral behavior of the newly discovered neutron star X-ray binary Swift J1858.6-0814

ABSTRACT We report on the NuSTAR observation of the newly discovered neutron star X-ray binary Swift J1858.6-0814 taken on 23rd March 2019. The light curve of the source exhibits several large flares during some time intervals of this observation. The source is softer in the high-intensity interval where the large flaring activity mainly occurs. We perform time-resolved spectroscopy on the source by extracting spectra for two different intensity intervals. The source was observed with a 3 − 79keV luminosity of ∼9.68 × 1036 ergs/s and ∼4.78 × 1036 ergs/s for high and low-intensity interval, respectively assuming a distance of 15 kpc. We find a large value of the absorbing column density ($\rm {N_{H}}\sim 1.1\times 10^{23}$ cm−2), and it appears to be uncorrelated with the observed flux of the source. Each spectrum shows evidence of Fe Kα emission in the 5 − 7keV energy band, an absorption edge around ∼7 − 8keV, and a broad Compton hump above 15keV, indicating the presence of a reflection spectrum. The observed features are well explained by the contribution of a relativistic reflection model and a partially covering absorption model. From the best-fit spectral model, we found an inner disc radius to be $4.87_{-0.96}^{+1.63}\,\,R_{ISCO}$ (for the high-intensity interval) and $5.68_{-2.78}^{+9.54}\,\,R_{ISCO}$ (for the low-intensity interval), indicating a significant disc truncation. The disk inclination is found to be relatively low, i &amp;lt; 330. We further place an upper limit on this source’s magnetic field strength considering the disc is truncated at the magnetospheric radius.

Multiwavelength Characterization of the High-mass X-Ray Binary Population of M33

We present multiwavelength characterization of 65 high-mass X-ray binary (HMXB) candidates in M33. We use the Chandra ACIS survey of M33 (ChASeM33) catalog to select hard X-ray point sources that are spatially coincident with UV-bright point-source optical counterparts in the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region catalog, which covers the inner disk of M33 at near-IR, optical, and near-UV wavelengths. We perform spectral energy distribution fitting on multiband photometry for each point-source optical counterpart to measure its physical properties including mass, temperature, luminosity, and radius. We find that the majority of the HMXB companion star candidates are likely B-type main-sequence stars, suggesting that the HMXB population of M33 is dominated by Be X-ray binaries (Be-XRBs), as is seen in other Local Group galaxies. We use spatially resolved recent star formation history maps of M33 to measure the age distribution of the HMXB candidate sample and the HMXB production rate for M33. We find a bimodal distribution for the HMXB production rate over the last 80 Myr, with a peak at ∼10 and ∼40 Myr, which match theoretical formation timescales for the most massive HMXBs and Be-XRBs, respectively. We measure an HMXB production rate of 107–136 HMXBs/(M ⊙ yr−1) over the last 50 Myr and 150–199 HMXBs/(M ⊙ yr−1) over the last 80 Myr. For sources with compact object classifications from overlapping NuSTAR observations, we find a preference for giant/supergiant companion stars in black hole HMXBs and main-sequence companion stars in neutron star HMXBs.

RX J0440.9+4431: another supercritical X-ray pulsar

ABSTRACT In the beginning of 2023, the Be transient X-ray pulsar RX J0440.9+4431 underwent a first-ever giant outburst observed from the source peaking in the beginning of February and reaching peak luminosity of ≈4.3 × 1037 erg s−1. Here, we present the results of a detailed spectral and temporal study of the source based on NuSTAR, Swift, INTEGRAL, and NICER observations performed during this period and covering wide range of energies and luminosities. We find that both the pulse profile shape and spectral hardness change abruptly around ≈2.8 × 1037 erg s−1, which we associate with a transition to supercritical accretion regime and erection of the accretion column. The observed pulsed fraction decreases gradually with energy up to 20 keV (with a local minimum around fluorescence iron line), which is unusual for an X-ray pulsar, and then rises rapidly at higher energies with the pulsations significantly detected up to ≈120 keV. The broad-band energy spectra of RX J0440.9+4431 at different luminosity states can be approximated with a two-hump model with peaks at energies of about 10–20 and 50–70 keV previously suggested for other pulsars without additional features. In particular, an absorption feature around 30 keV previously reported and interpreted as a cyclotron line in the literature appears to be absent when using this model, so the question regarding the magnetic field strength of the neutron star remains open. Instead, we attempted to estimate field using several indirect methods and conclude that all of them point to a relatively strong field of around B ∼ 1013 G.

Accretion properties and estimation of spin of galactic black hole candidate Swift J1728.9–3613with NuSTAR during its 2019 outburst

ABSTRACT Black hole X-ray binaries (BHXRBs) play a crucial role in understanding the accretion of matter onto a black hole. Here, we focus on exploring the transient BHXRB Swift J1728.9–3613 discovered by Swift/BAT and MAXI/GSC during its January 2019 outburst. We present measurements on its accretion properties, long time-scale variability, and spin. To probe these properties, we make use of several NICER observations and an unexplored data set from NuSTAR, as well as long-term light curves from MAXI/GSC. In our timing analysis, we provide estimates of the cross-correlation functions between light curves in various energy bands. In our spectral analysis, we employ numerous phenomenological models to constrain the parameters of the system, including flavours of the relativistic reflection model Relxill to model the Fe Kα line and the &amp;gt;15 keV reflection hump. Our analysis reveals that: (i) Over the course of the outburst, the total energy released was ∼5.2 × 1044 ergs, corresponding to roughly 90 per cent of the mass of Mars being devoured. (ii) We find a continuum lag of 8.4 ± 1.9 d between light curves in the 2–4 and 10–20 keV bands, which could be related to the viscous inflow time-scale of matter in the standard disc. (iii) Spectral analysis reveals a spin parameter of ∼0.6–0.7 with an inclination angle of ∼45°–70° and an accretion rate during the NuSTAR observation of ${\sim}17\ \hbox{per cent}\ L_{\rm Edd}$.

Rapid Variability of the Accretion Disk Wind in the Narrow-line Seyfert 1, PG 1448+273

PG 1448+273 is a luminous, nearby (z = 0.0645), narrow-line Seyfert 1 galaxy, which likely accretes close to the Eddington limit. XMM-Newton observations of PG 1448+273 in 2017 revealed the presence of an ultrafast outflow, as seen through its blueshifted iron K absorption profile, with an outflow velocity of about 0.1 c. Here, the first NuSTAR observation of PG 1448+273, performed in 2022 and coordinated with XMM-Newton, is presented, which shows remarkable variability of its ultrafast outflow. The average count rate is a factor of 2 lower during the last 60 ks of the NuSTAR observation, where a much faster component of the ultrafast outflow was detected with a terminal velocity of 0.26 ± 0.04 c. This is significantly faster than the outflow component that was initially detected in 2017, when overall PG 1448+273 was observed at a lower X-ray flux, and which implies an order of magnitude increase in the wind kinetic power between the 2017 and 2022 epochs. Furthermore, the rapid variability of the ultrafast outflow in 2022, on timescales down to 10 ks, suggests we are viewing through a highly inhomogeneous disk wind in PG 1448+273, where the passage of a denser wind clump could account for the increase in obscuration in the last 60 ks of the NuSTAR observation.

Dimming of Continuum Captured in Mrk 841: New Clues on the Nature of the Soft X-Ray Excess

We report on a remarkable change in the spectral energy distribution (SED) of Mrk 841, providing new insights on how the soft X-ray excess emission in active galactic nuclei (AGNs) is produced. By Swift monitoring of a sample of Seyfert-1 galaxies, we found an X-ray spectral hardening event in Mrk 841. We thereby triggered our XMM-Newton, NuSTAR, and Hubble Space Telescope observations in 2022 to study this event. Our previous investigations of such events in other AGNs had shown that they are caused by obscuring winds. However, the event in Mrk 841 has different spectral characteristics and origin. We find it is the soft X-ray excess component that has become dimmer. This is, importantly, accompanied by a similar decline in the optical/UV continuum, suggesting a connection to the soft X-ray excess. In contrast, there is relatively little change in the X-ray power law and the reflection components. Our SED modeling suggests that the soft X-ray excess is the high-energy extension of the optical/UV disk emission, produced by warm Comptonization. We find the temperature of the disk dropped in 2022, explaining the observed SED dimming. We then examined the Swift data, taken over 15 yr, to further decipher the UV and X-ray variabilities of Mrk 841. A significant relation between the variabilities of the X-ray spectral hardness and that of the UV continuum is found, again suggesting that the soft excess and the disk emission are interlinked. This is readily explicable if the soft excess is produced by warm Comptonization.

The Large Deficit of HMXB Emission from Luminous Infrared Galaxies: The Case of the Circumnuclear Starburst Ring in NGC 7552

Luminous infrared galaxies (LIRGs), the most extreme star-forming galaxies in the nearby (D < 30 Mpc) universe, show a notable X-ray emission deficiency (up to a factor of ∼10) compared with predictions from scaling relations of galaxy-wide high-mass X-ray binary (HMXB) luminosity with star formation rate. In the nearby (≈20 Mpc) LIRG NGC 7552, the majority of the IR emission originates in a circumnuclear starburst ring, which has been resolved into several discrete knots of star formation. We present results from recent Chandra observations of NGC 7552, which reveal significant deficits in the 2–7 keV X-ray luminosities from two of the most powerful star-forming knots. We hypothesize that the expected luminous HMXB populations in these knots are either (1) obscured by very large column densities or (2) suppressed due to the knots having relatively high metallicity and/or very young ages (≲5 Myr). We distinguish between these possibilities using data from recent NuSTAR observations, whose sensitivity above 10 keV is capable of uncovering heavily obscured HMXB populations, since emission at these energies is more immune to absorption effects. We find no evidence of a heavily obscured HMXB population in the central region of NGC 7552, suggesting suppressed HMXB formation. We further show that metallicity-dependent scaling relations cannot fully account for the observed deficit from the most powerful star-forming knots or the central region as a whole. Thus, we suggest that recent bursts in local star formation activity likely drive the high L IR within these regions on timescales ≲5 Myr, shorter than the timescale required for the formation of HMXBs.

Drop in the Hard Pulsed Fraction and a Candidate Cyclotron Line in IGR J16320–4751 Seen by NuSTAR

We report on a timing and spectral analysis of a 50 ks NuSTAR observation of IGR J16320−4751 (= AX J1631.9−4752), a high-mass X-ray binary hosting a slowly rotating neutron star. In this observation from 2015, the spin period was 1308.8 ± 0.4 s giving a period derivative s s−1 when compared with the period measured in 2004. In addition, the pulsed fraction decreased as a function of energy, as opposed to the constant trend that was seen previously. This suggests a change in the accretion geometry of the system during the intervening 11 yr. The phase-averaged spectra were fit with the typical model for accreting pulsars: a power law with an exponential cutoff. This left positive residuals at 6.4 keV attributable to the known iron Kα line, as well as negative residuals around 14 keV from a candidate cyclotron line detected at a significance of 5σ. We found no significant differences in the spectral parameters across the spin period, other than the expected changes in flux and component normalizations. A flare lasting around 5 ks was captured during the first half of the observation, where the X-ray emission hardened and the local column density decreased. Finally, the binary orbital period was refined to 8.9912 ± 0.0078 days thanks to Swift/BAT monitoring data from 2005–2022.

MASTER OT J030227.28+191754.5: A Dwarf Nova at a Massive Oxygen–Neon White Dwarf System?

We present timing and spectral analysis results of the NICER and NuSTAR observations of the dwarf nova MASTER OT J030227.28+191754.5 during the 2021–2022 outburst. The soft X-ray component was found to be dominated by blackbody radiation with a temperature of ∼30 eV and also showed prominent oxygen and neon emission lines. The blackbody luminosity exceeded 1034 erg s−1, which is consistent with theoretical predictions, and then decreased more than an order of magnitude in 3.5 days. The inferred abundances of oxygen and neon in the optically thin coronal region surrounding the central white dwarf (WD) are several times higher than the respective solar values. Although inconclusive, the abundance enrichment may originate from the WD, indicating that it may be mainly composed of oxygen and neon. Assuming that the blackbody radiation comes from the belt-shaped boundary layer between the WD and the accretion disk, we estimated the WD radius to be (2.9 ± 1.1) × 108 cm, which corresponds to the WD mass range of 1.15–1.34 M ⊙. If the accretion continues for another ∼1 Gyr, the WD may experience an accretion-induced collapse into a neutron star and form a so-called black widow pulsar system.

Early Hard X-Rays from the Nearby Core-collapse Supernova SN 2023ixf

We present NuSTAR observations of the nearby SN 2023ixf in M101 (d = 6.9 Mpc) that provide the earliest hard X-ray detection of a nonrelativistic stellar explosion to date at δ t ≈ 4 days and δ t ≈ 11 days. The spectra are well described by a hot thermal bremsstrahlung continuum with T > 25 keV shining through a thick neutral medium with a neutral hydrogen column that decreases with time (initial N Hint = 2.6 × 1023 cm−2). A prominent neutral Fe Kα emission line is clearly detected, similar to other strongly interacting supernovae (SNe) such as SN 2010jl. The rapidly decreasing intrinsic absorption with time suggests the presence of a dense but confined circumstellar medium (CSM). The absorbed broadband X-ray luminosity (0.3–79 keV) is L X ≈ 2.5 × 1040 erg s−1 during both epochs, with the increase in overall X-ray flux related to the decrease in the absorbing column. Interpreting these observations in the context of thermal bremsstrahlung radiation originating from the interaction of the SN shock with a dense medium we infer large particle densities in excess of n CSM ≈ 4 × 108 cm−3 at r < 1015 cm, corresponding to an enhanced progenitor mass-loss rate of M ⊙ yr−1 for an assumed wind velocity of v w = 50 km s−1.

Study of recently discovered Be/X-ray pulsar MAXI J0655-013 using NuSTAR

Abstract We study the recently discovered Be/X-ray pulsar MAXI J0655–013 using the 2022 NuSTAR observations. The paper is the first detailed study of the timing and spectral properties of the source. The pulse profiles of the pulsar vary with energy. The pulsed fraction is found to increase monotonically with energy. In between the two NuSTAR observations, a large spin-up rate of ∼ -1.23 s d−1 is observed, which can be due to large spin-up torque acting on the pulsar during an outburst. Such a large spin-up rate is observed for the first time in an X-ray pulsar during an outburst. The variation of the spin period with time can be employed to obtain the orbital parameters of the binary system, and we found the orbital period to be ∼ 27.9 d. The second NuSTAR observation is done in a low luminosity state (LX ∼ 3.9 × 1034 erg s−1 ). We have detected the pulsation of the pulsar in such a low luminosity state. In such a low luminosity state, the pulsar MAXI J0655–013 might be accreting from the cold disk.

NuSTAR investigation of X-ray variability and hard X-ray spectral properties in IGR J16320-4751 and IGR J16479-4514

Abstract We present the results obtained from a comprehensive timing and spectral study of two high mass X-ray binary sources using NuSTAR observations. These two sources, IGR J16320-4751 and IGR J16479-4514 were discovered by INTEGRAL and have been characterizated for the first time in the hard X-ray band (beyond 10 keV) in this work. In these sources, we observe the occurrence of intense X-ray flares, with average luminosities exceeding 1036 erg s−1. Our analysis reveals that these flares can be described consistently in the quasi-spherical accretion regime. The orbital phase of the first flare in NuSTAR observation of IGR J16479-4514 matches with the orbital phases of previous flares (φ = 0.35) in this source detected by other telescopes. We conclude that this flare occurs as a result of the periastron passage of the neutron star, rather than due to the presence of a corotating interaction region (CIR). Furthermore, from the energy-resolved pulse profile analysis of IGR J16320-4751, we find that the pulse fraction is lower in hard X-rays compared to the soft X-rays. We present the hard X-ray spectral parameters of these two sources using several standard spectral model components. We do not detect a cyclotron absorption feature in either target. We provide estimates to the surface magnetic field strength of NS in IGR J16320-4751 using two indirect methods. Lastly, we observe spectral hardening during flaring segments compared to the off-flaring segments which indicates that comptonization is more effective during the flaring segments.

Observing the onset of the accretion wake in Vela X-1

High-mass X-ray binaries (HMXBs) offer a unique opportunity to investigate accretion onto compact objects and the wind structure in massive stars. A key source for such studies is the bright neutron star HMXB Vela X-1 whose convenient physical and orbital parameters facilitate analyses and in particular enable studies of the wind structure in HMXBs. Here, we analyse simultaneous XMM-Newton and NuSTAR observations at ϕorb ≈ 0.36–0.52 and perform time-resolved spectral analysis down to the pulse period of the neutron star based on our previous NuSTAR-only results. For the first time, we are able to trace the onset of the wakes in a broad 0.5–78 keV range with a high-time resolution of ~283 s and compare our results with theoretical predictions. We observe a clear rise in the absorption column density of the stellar wind NH,1 starting at orbital phase ~0.44, corresponding to the wake structure entering our line of sight towards the neutron star, together with local extrema throughout the observation, which are possibly associated with clumps or other structures in the wind. Periods of high absorption reveal the presence of multiple fluorescent emission lines of highly ionised species, mainly in the soft-X-ray band between 0.5 and 4 keV, indicating photoionisation of the wind.

A NuSTAR and Chandra Investigation of the Misaligned Outflow of PSR J1101–6101 and the Lighthouse Pulsar Wind Nebula

PSR J1101–6101 is an energetic young pulsar that powers the remarkable Lighthouse pulsar wind nebula (PWN). The pulsar belongs to the rare type of radio- and gamma-ray-quiet pulsars that are bright in hard X-rays. Moreover, the Lighthouse PWN is remarkable for its misaligned outflow (which gave rise to the PWN’s nickname). Also known as “pulsar filaments,” these collimated parsec-scale X-ray structures have been recently discovered in the vicinity of a handful of fast-moving pulsars, and they appear unaffected by the ram pressure that confines pulsar tails. We report on NuSTAR observations of PSR J1101–6101 and its misaligned outflow—the first observation of such a structure above ∼10 keV. We detect the outflow up to 25 keV, spatially resolve its spectral evolution with distance from the pulsar, find unambiguous evidence of spectral cooling with distance from the pulsar, and infer physical properties of the particles and magnetic field in the outflow. We also reanalzye archival Chandra data and discuss the outflow’s small-scale structure. We detect pulsations from PSR J1101–6101 up to 20 keV, present the X-ray pulse profile, confirm its period derivative, and perform phase-resolved spectroscopy. Finally, we discuss the X-ray source 2CXO J110158.4–605649 = 2XMM J110158.5–605651 (a serendipitously observed blazar) and suggest it may be the X-ray counterpart to the GeV source 4FGL J1102.0–6054.